The present invention relates to novel triarylmethyl free radicals and their use as image enhancing agents in magnetic resonance imaging (MRI), in particular to their use in electron spin resonance enhanced magnetic resonance imaging (OMRI) of a sample (for example a human or animal body) for determining the oxygen concentration of said sample.
MRI is a diagnostic technique that has become particularly attractive to physicians as it is non-invasive and does not involve exposing the patient under study to potentially harmful radiation (eg. X-rays). The technique, however, suffers from inter alia the problem of achieving effective contrast in the magnetic resonance (MR) images between tissue types having the same or closely similar imaging parameters.
Electron spin resonance enhanced MRI, referred to herein as OMRI(Overhauser MRI) but also referred to in earlier publications as ESREMRI or PEDRI, is a particular form of MRI in which enhancement of the magnetic resonance signals from which the images are generated is achieved by virtue of the dynamic nuclear polarization (the Overhauser effect) that occurs on VHF stimulation of an esr transition in a paramagnetic material, generally a persistent free radical, in the subject under study. Magnetic resonance signal enhancement may be by a factor of a hundred or more thus allowing OMRI images to be generated rapidly and with relatively low primary magnetic fields.
OMRI techniques have been described by several authors, notably Leunbach, Lurie, Ettinger, Grucker, Ehnholm and Sepponen, for example in EP-A-296833, EP-A-361551, WO-A-90/13047, J. Mag. Reson. 76:366-370(1988), EP-A-302742, SMRM 9:619(1990), SMRM 6:24(1987), SMRM 7:1094(1988), SMRM 8:329(1989), U.S. Pat. No. 4,719,425, SMRM 8:816(1989), Mag. Reson. Med. 14:140-147(1990), SMRM 9:617(1990), SMRM 9:612(1990), SMRM 9:121(1990), GB-A-2227095, DE-A-4042212 and GP-A-2220269.
In the basic OMRI technique, the imaging sequence involves initially irradiating a subject placed in a uniform magnetic field (the primary field B.sub.0) with radiation, usually VHF radiation, of a frequency selected to excite a narrow linewidth esr transition in a paramagnetic enhancement agent (hereinafter "an OMRI contrast agent") which is in or has been administered to the subject. Dynamic nuclear polarization results in an increase in the population difference between the excited and ground nuclear spin states of the imaging nuclei, i.e. those nuclei, generally protons, which are responsible for the magnetic resonance signals. Since MR signal intensity is proportional to this population difference, the subsequent stages of each imaging sequence, performed essentially as in conventional MRI techniques, result in larger amplitude MR signals being detected and more effective contrast.
A number of oxygen free radicals that is to say radicals where the unpaired electron or electrons are associated with the oxygen atom have been proposed as OMRI contrast agents including for example nitroxide stable free radicals, chloranil semiquinone radical and Fremy's salt (U.S. Pat. No. 4,984,573) and deuterated stable free radicals, in particular deuterated nitroxide stable free radicals (WO-A-90/00904).
In WO-A-91/12024, Nycomed Innovation AB proposed persistant carbon free radicals, i.e. radicals (e.g. triarylmethyl radicals) in which the unpaired electron(s) are primarily associated with carbon atoms, for use as OMRI contrast agents.
In WO-A-96/39367, Nycomed Imaging AS proposed various sulphur-based triarylmethyl radicals for use as OMRI contrast agents.
In any OMRI experiment under ambient conditions, paramagnetic oxygen will have a finite effect on the spin system present. Generally speaking, this may be dismissed as a secondary effect when compared to the primary interaction of the radical electron spin and the nuclear spin system. Nonetheless, it has been proposed that this effect may be used to determine oxygen concentration within a sample. Research has concentrated particularly on the use of nitroxide spin labels; radicals which suffer the inherent disadvantage of having broad linewidth esr resonances and therefore low sensitivity to the effects of oxygen. Thus, to date, the effect of oxygen has been recognised only in a qualitative sense and any attempt to attach a quantitative significance to the oxygen effect has failed. Moreover, in general, non-invasive techniques for oxygen determination have been slow to develop and typically are not suited to the study of tissues lying deep beneath the surface of a sample.
For example, Grucker et al (MRM, 34:219-225(1995)) reported a method for calculating oxygen concentration by measuring the Overhauser effect attributable to a nitroxide radical and relating the non-linear effect of oxygen on the Overhauser Factor to its concentration. This involved taking two images, one on-resonance and one off-resonance, and using a first order approximation to arrive at the oxygen concentration. However, Grucker observed that the correlation between actual and calculated oxygen concentration was poor and therefore that the method was inherently inaccurate. This was attributed to the large number of parameters involved in the calculation.
Ehnholm (U.S. Pat. No. 5,289,125) proposed an OMRI technique in which signals from a paramagnetic material were detected under at least two different sets of operating parameters whereby to generate images of various physical, chemical or biological parameters. While oxygen tension was one of several such parameters, Ehnholm did not demonstrate the use of the technique to quantitate dissolved oxygen.